1. Field of the Invention
The present invention relates to a laser mask and a method of crystallization using the same, and more particularly to, a laser mask and a method of crystallization using the same that can produce a polycrystalline silicon thin film having uniform crystallization characteristics.
2. Discussion of the Related Art
Recently, due to the needs for information displays, especially for portable information displays, thin film type flat panel display (FPD) devices have been actively being researched and commercialized such that the cathode ray tubes (CRT) are being replaced. Of these flat panel display devices, liquid crystal display (LCD) devices are widely used for notebook computers and desktop monitors, due to their excellent resolution, color rendering capability and picture quality.
An active matrix (AM) driving method, a typical driving method used for the LCD devices, drives each of the pixels of an LCD device using an amorphous silicon thin film transistor (a-Si TFT) as a switching element. The a-Si TFT technique was described by English LeComber et al. in 1979, and was commercialized as a three-inch liquid crystal portable television in 1986. Recently, a TFT-LCD device with a display area of more than 50 inches has been developed. However, the field effect mobility of the a-Si TFT is about 1 cm2/Vsec, which prevents its use in peripheral circuits that apply signals to the pixels, because the peripheral circuits generally operate at more than 1 MHz. Accordingly, researches for simultaneously forming switching transistors in a pixel region and peripheral circuits in a driving circuit region on a glass substrate using a polycrystalline silicon (poly-Si) TFT having a field effect mobility greater than that of the a-Si TFT have been actively pursued.
The poly-Si TFT has been applied to small flat panel displays, such as the eyepiece of camcorders, since an LCD color television was developed in 1982. Such a TFT has low photosensitivity and high field effect mobility, and it can be directly fabricated on a substrate to form driving circuits. Increased mobility can increase the operation frequency of the driving circuits. The frequency capability of the driving circuits determines the number of the pixels that, can be driven while maintaining adequate display capability. More specifically, the increased frequency decreases the charging time of a signal applied to a pixel such that distortion of the signal is decreased and the picture quality increases.
The poly-Si TFT can be fabricated by directly depositing a polycrystalline silicon thin film on a substrate or by depositing an amorphous silicon thin film that is then crystallized by a thermal process. To use a cheap glass as a substrate, low temperature processes are required, and, to use the poly-Si TFT for driving circuits, a method for increasing the field effect mobility is required. In general, thermal processing methods for crystallizing an amorphous silicon thin film are the solid phase crystallization (SPC) method and the excimer laser annealing (ELA) method.
The SPC method forms a polycrystalline silicon thin film at a low temperature of approximately 600° C. In this method, a polycrystalline silicon thin film is formed by depositing an amorphous silicon thin film on a glass substrate having a low melting point and then by performing a slow heating process at approximately 600° C. for up to tens of hours. A polycrystalline silicon thin film obtained by the SPC method has comparatively large-size grains of about several μm (micrometers). However, there are many defects in the grains. Although not as bad as grain boundaries in a poly-Si TFT, these defects affect negatively on the performance of a poly-Si TFT.
The excimer laser annealing method is a typical method of fabricating a poly-Si TFT at a low temperature. The excimer laser crystallizes an amorphous silicon thin film by irradiating a high energy laser beam onto the amorphous silicon thin film for a time of ten nanoseconds. In this method, the amorphous silicon is melted and crystallized in a very short time, so that the glass substrate is not damaged. A polycrystalline silicon thin film fabricated by the excimer laser method also has excellent electrical characteristics, compared to a poly-Si thin film fabricated by a general thermal processing method. For example, a field effect mobility of a poly-Si TFT fabricated by the excimer laser method is more than 100 cm2/Vsec, whereas a field effect mobility of an a-Si TFT is 0.1˜0.2 cm2/Vsec and a field effect mobility of a poly-Si TFT fabricated by a general thermal processing method is 10˜20 cm2/Vsec (IEEE Trans. Electron Devices, vol. 36, no. 12, p. 2868, 1989).
A crystallization method using a laser will now be described in detail. FIG. 1 is a graph illustrating a relationship between a grain size of a polycrystalline silicon thin film and an energy density of a laser used to form the polycrystalline silicon thin film.
As shown in FIG. 1, in the first and second regions I and II, as the energy density increases, the grain size of the polycrystalline silicon thin film increases, as discussed in IEEE Electron Device Letters, DEL-7, 276, 1986. However, in the third region III, when the energy density becomes higher than a specific energy density Ec, the grain size of the crystallized polycrystalline silicon thin film decreases drastically. That is, according to the graph shown in FIG. 1, the crystallization mechanism for the silicon thin film becomes different when the energy density is higher than a specific energy density Ec.
FIGS. 2A to 2C, 3A to 3C and 4A to 4C are sectional views illustrating silicon crystallization mechanisms according to the laser energy densities of FIG. 1. That is, they illustrate sequential crystallization process according to each laser energy density. A crystallization mechanism of amorphous silicon by a laser annealing is influenced by many factors, such as laser irradiation conditions including laser energy density, irradiation pressure, substrate temperature, and physical/geometrical characteristics including absorption coefficient, thermal conductivity, mass, impurity containing degree and amorphous silicon layer thickness.
First, as shown in FIGS. 2A to 2C, the first region (I) of FIG. 1 is a partial melting region, and an amorphous silicon thin film 12 is crystallized only down to the dotted line and a size of a grain G1 formed at this time is about hundreds Å. When a laser beam is irradiated on the amorphous silicon thin film 12 on a substrate 10 where a buffer layer 11 is formed, the amorphous silicon thin film 12 is melted. At this time, because strong laser energy is irradiated directly at a surface of the amorphous silicon thin film 12 and relatively weak laser energy is irradiated at a lower portion of the amorphous silicon thin film 12, the amorphous silicon thin film 12 is melted only down to a certain portion, thereby achieving a partial crystallization.
Typically, in the laser crystallization method, crystals grow through the processes of primary melting in which a surface layer of an amorphous silicon thin film is melted by a laser irradiation, secondary melting in which a lower portion of the amorphous silicon thin film is melted by the latent heat generated during the solidification of the melted silicon, and the solidification of the lower layer. These crystal growth processes will be explained in more detail.
An amorphous silicon thin film on which a laser beam is irradiated has a melting temperature of more than 1000° C. and primarily melts into a liquid state. Because there is a great temperature difference between the surface melted layer and the lower silicon and substrate, the surface melted layer cools fast until solid phase nucleation and solidification are achieved. The surface layer remains melted until the solid phase nucleation and solidification are completed. The melting state lasts for a long time when the laser energy density is high or thermal emission to the outside is low. Because the surface layer melts at a lower temperature, for example 1000° C., than the melting temperature of 1400° C. for crystalline silicon, the surface layer cools and maintains a super-cooled state where the temperature is lower than the phase transition temperature.
The greater the super-cooling state is, that is, the lower the melting temperature of the thin film or the faster the cooling speed is, the greater the nucleation rate is at the time of the solidification such that fine crystals grow during the solidification. When the solidification starts as the melted surface layer cools, crystals grow in an upward direction from a crystal nucleus. At this time, latent heat is generated during the phase transition of the melted surface layer from liquid state to solid state, and thus the secondarily melting begins where the lower amorphous silicon thin film melts. Then, the solidification of the lower amorphous silicon thin film occurs. At this time, the nucleus generation rate of the lower second melted layer increases, because the lower amorphous silicon thin film is more super-cooled than the first melted layer. Thus, the crystal size resulting from the second melted layer is smaller. Accordingly, the cooling speed of the solidification has to be reduced to improve the crystalline characteristics. Cooling speed can be reduced by restraining absorbed laser energy from being emitted to the outside. Examples of the restraining method are heating the substrate, double beam irradiation, or inserting a buffer insulating layer between the substrate and the amorphous silicon layer.
FIGS. 3A to 3C are sectional views illustrating the silicon crystallization mechanism of the second region (II) of FIG. 1, in which the second region (II) represents a near-completely crystallized region.
Referring to FIGS. 3A to 3C, a polycrystalline silicon thin film has relatively large grains 30A-30C of about 3000 to 4000 Å formed down to the interface of the lower buffer layer 11. When a nearly complete melting energy, not a complete melting energy, is irradiated on the amorphous silicon thin film 12, almost all of the amorphous silicon thin film 12 close to the buffer layer 11 melts. At this time, solid seeds 35 that have not been melted at the interface between the melted silicon thin film 12′ and the buffer layer 11 work as a crystallization nucleus to induce a lateral growth, thereby forming the relatively large grains 30A-30C (J. Appl. Phys. 82, 4086). However, because this crystallization only occurs when the laser energy is such that the solid seeds 35 remain on the interface with the buffer layer 11, the process margin is very limited. In addition, because the solid seeds 35 are generated non-uniformly, the crystallized grains 30A-30C of the polycrystalline silicon thin film have different crystallization directions, thereby resulting in non-uniform crystallization characteristics.
FIGS. 4A to 4C are sectional views illustrating the silicon crystallization mechanism of the third region (III) of FIG. 1 corresponding to a completely crystallized region.
Referring to FIGS. 4A to 4C, very small grains 30 are irregularly formed with a energy density corresponding to the third region (III). When the laser energy density becomes higher than a specific energy density Ec, sufficient energy is applied enough to completely melt the amorphous silicon thin film 12, leaving no solid seed that may be grown to grains. Thereafter, the silicon thin film 12′ which has been melted upon receiving the laser beam of the strong energy undergoes a rapid cooling process, which generates a plurality of uniform nuclei 35 and thus fine grains 30.
Meanwhile, an excimer laser annealing method employing a pulse-type laser is typically used for the laser crystallization, and a sequential lateral solidification (SLS) method, which shows remarkable improvement of crystallization characteristics by growing grains in a horizontal direction, has recently been proposed and studied widely.
The sequential lateral solidification (SLS) utilizes the fact that grains grow laterally from an interface between liquid phase silicon and solid phase silicon (Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proco. Vol. 452, 956 to 957, 1997). In this method, grains grow laterally with a predetermined length by controlling the laser energy density and irradiation range of a laser beam, thereby increasing the size of silicon grains.
This SLS is one example of lateral solidification (LS), and the crystallization mechanism with respect to the LS will now be described with reference to the accompanying drawings. FIGS. 5A to 5C are sectional views sequentially illustrating a crystallization process according to a related art general lateral crystallization.
Referring to FIG. 5A, when a laser having an energy density in the third region (III) of FIG. 1, the energy density capable of completely melting an amorphous silicon thin film 112, is irradiated onto a portion of an amorphous silicon thin film 112, the portion of the amorphous silicon film completely melts. A patterned mask can be employed to form a laser irradiated region and a laser non-irradiated region. At this time, as shown in FIGS. 5B and 5C, because the laser has sufficient energy, the amorphous silicon thin film 112 irradiated by the laser can be completely melted. However, the laser beam is irradiated with certain intervals on the amorphous silicon thin film 112, crystals grow from the interface between the silicon thin film 112 of the laser non-irradiated region (solid phase) and the melted silicon thin film 112′ (liquid phase).
Thus, the interface provides nuclei for this crystallization. In other words, immediately after the laser beam is irradiated, the melted silicon thin film 112′ cools from the left/right surfaces, the interfaces of the laser non-irradiated region. This is because the solid phase amorphous silicon thin film 112 has higher heat conductivity than the buffer layer 111 or the glass substrate 110 below the silicon thin films 112 and 112′. Accordingly, the melted silicon thin film 112′ first reaches a nucleus formation temperature at the interface between the horizontal solid phase and the liquid phase, rather than at the central portion, thereby forming a crystal nucleus at the corresponding portion. After the crystal nucleus is formed, grains 130A and 130B horizontally grow from a low-temperature side to a high-temperature side, that is, from the interface to the central portion. Due to the lateral crystallization, large-size grains 130A and 130B can be formed, and because the process is performed with the energy density of the third region, the process margin is not limited, compared to other regions.
However, the SLS has the following problems. That is, the crystallization is performed by infinitesimally and repeatedly moving the mask or the stage in order to increase the size of the grains. As a result, the process time for crystallizing a large-size amorphous silicon thin film is lengthened and the process yield becomes low.